The invention relates to loudspeakers and, more particularly, to bending wave panel-form loudspeakers, e.g. of the kind described in International patent application WO97/09842 and U.S. counterpart application Ser. No. 08/707,012, filed Sep. 3, 1996. Such loudspeakers are known as xe2x80x9cdistributed modexe2x80x9d or xe2x80x9cDMxe2x80x9d loudspeakers, or simply as DML.
Bending wave panels emit acoustic radiation from both sides,. i.e. are bi-polar, and to allow easier positioning of the loudspeaker in a room, e.g. against a wall, the rear radiation from the panel can be blocked by mounting the panel in a sealed box or enclosure with flexible suspension or surround material around the panel perimeter. This prevents any destructive interference from reflecting surfaces to the rear of the panel. The panel suspension or surround material may be a strip of flexible foam rubber or plastics with adhesive tape on both sides so that it can be fixed to both the panel and the box.
However, in such an arrangement standing waves may be set up in the box or enclosure which can adversely affect the frequency response of the panel. These standing waves can be damped by filling the box or enclosure with an acoustic absorber, e.g. of soft foam which can contact either the bending wave panel or the back of the box or enclosure. Flexible polyester or polyether foams are suitable for this application. A known setup for a bending wave panel of the DML kind in an enclosure uses a strip of flexible foam as a suspension or surround and uses a separate acoustic absorber in the enclosure.
According to the present invention, there is provided a loudspeaker comprising a bending wave panel in an enclosure, wherein the panel surround suspension system and the acoustic absorber are integrated. A soft foam used as the acoustic absorber may thus be compressed to the required thickness around the perimeter of the panel to form the flexible surround, whereas the central region of the foam remains uncompressed allowing it to form an effective acoustic absorber. In other words, the density of the foam in the perimeter region is higher than that in the central region.
According to another aspect of the invention, there is provided a method of making a loudspeaker sub-assembly having a bending wave panel, a flexible surround suspension for the panel and an acoustic absorber for filling the space between the panel and an enclosure to which the panel will be mounted via the suspension. The method comprises the steps of applying a layer of adhesive to one face of the panel along the perimeter thereof; placing a piece of uncompressed foam on the one face of the panel so as to cover the one face and contact the adhesive; applying heat and pressure to the panel and the foam along the perimeter thereof to compress the foam and cure the adhesive so as to form a surround suspension that is bonded to the panel and is denser than the acoustic absorber foam bounded by the surround suspension; and removing the heat and pressure from the panel and the foam.
According to yet another aspect of the invention, there is provided a method of making a loudspeaker having a sub-assembly made as above, comprising the further step of assembling the panel, with integral surround suspension and acoustic absorber, to the enclosure.